Redundant Power Supply (RPS) systems provide a backup source of power for a system. If the system's primary power source fails or if the system otherwise loses power, the redundant power supply quickly turns on an auxiliary power supply and couples the auxiliary power supply to the system. Redundant power supply systems are designed to provide auxiliary power supply to the system quickly enough that the system can continue operating throughout the transition from primary to auxiliary power. Thus, a redundant power supply can provide protection against system failure or outage.
When a system that has never been powered up or whose voltage has already fallen below a certain level is coupled to the output port of a redundant power supply, very high current can flow. This current flows until the input capacitors within the system that is being powered by the redundant power supply have an opportunity to fully charge. This initial surge of current is referred to as inrush current.
High inrush current is undesirable for several reasons. Excessive inrush current can damage system components, cause various system errors, and reduce the available supply voltage. In some situations, high inrush current can even cause brownout situations in which the redundant power supply needs to shut down and restart.
Techniques to limit inrush current are often impractical. For example, many inrush-limiting techniques involve providing current measurement feedback to the redundant power supply controller. Unfortunately, current measurement feedback is often too expensive to be implemented in a cost-effective manner. Additionally, the voltage drop across the current sense element used with current measurement feedback is often undesirable. Accordingly, new techniques for reducing inrush current are desirable.
While the invention is susceptible to various modifications and alternative forms, specific embodiments are provided as examples in the drawings and detailed description. It should be understood that the drawings and detailed description are not intended to limit the invention to the particular form disclosed. Instead, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.